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iS               PRACTICAL  ORGANIC CHEMISTRY

azotometer by slipping out the cork from the front of the com-
bustion tube* and hang a thermometer beside it. Do not,
however, stop the flow of carbon dioxide until the tube is nearly
cold. In this way, the copper spiral remains quite bright and
may be used for a second determination without being
reduced.

When the azotometer has stood for an hour in a cool place,
adjust the level by raising the reservoir so that the liquid in the
tube and reservoir stand at the same height. Read off the
volume, and at the same time note the temperature and the
barometric pressure.

The percentage of nitrogen may be calculated as follows : 

v is the observed volume of nitrogen.
B is the height of the barometer in mm.
/ is the temperature.

/is the vapour tension of the potash solution, which may be
taken to be equal to that of water without serious error.

The volume corrected to o and 760 mm. will be given by
the following expression : 

(273-{-/)76o

As the weight of r c.c. of nitrogen at o and 760 mm is 0*00126
gram, the percentage weight of nitrogen will be given by the
expression

)       o'oor26x roo

where <w is the weight of substance taken.

Example.o'2o6 gram of acetanilide gave iS'S c.c. of moist
X at if and 756 mm. [/at 17== 14*5 mm.]

18-8 x 273 x (756-14-5) x 0-126 =        fi       cent

(273 4-17) x 760 x o'2o6
Calculated for CSH9ON ; N = 10-37 per cent.

Instead of collecting the gas over dilute potash solution, it is often
customary to use a very strong solution consisting of equal weights of
potash and water. The vapour tension is practically nil* Or, again,
the nitrogen may be transferred to a graduated tube standing over

r